net: minor optimization in qdisc_qstats_cpu_drop()

per_cpu_inc() is faster (at least on x86) than per_cpu_ptr(xxx)++;

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>